In aqueous environment, a steel structure corrodes because its interface potential is not in the metallic stability domain of Fe.

Its interface potential is in the stability domain of Fe ion (e.g. Fe).

E (volt)EcorFeFe stable 5

Cathodic protection decreases the interface potential of a structure down to a potential close to its equilibrium potential or in its stability domain.

E (V)Ecor,FeFe2EFe/FePrinciple cathodic protection of Fe 6

Immunity, passivation and active corrosion domains of Fe

Anodic ProtectionCathodic ProtectionAlteration of EnvironmentEcorEcorFe corrodes because its potential is in active corrosion domain (Fe stability domain) 7Due to the presence of Cl-In more realistic diagramsbaCathodic protection of steel/cast iron structures can be applied in environment containing aggressive ions (b). It is not recommended when the pH of environment lt 5 8

E (V)Ecor,FeAnodic pol. curveCathodic pol.curveFe2EFe/FeFe stableLog icorLog i0Log iPrinciple cathodic protection of Fe 9E (V)Actual pol. curveEcorEcorE1EFe/FeApplied protection current density i1Log iDecreasing potential from Ecor to E1 needs applied protection current density i1 10How does it work ?Anodic and cathodic sites will be formed on the unprotected pipe exposed in aqueous environment. Corrosion current will flow through environment from anodic to cathodic sites. Discharged current on anodic site is forced back into the steel pipe by supplying higher protection current density than that of the discharged current. 11

Interface potential of steel structures are decreased either by electrically connecting to sacrificial anodes or by using impressed current. Note that the corrosion potential of sacrificial anodes have to be significantly lower than the corrosion potential of steel structure.

Both methods have to result in the flow of electrons through wire (or other metallic connector) toward steel structure direct current has to flow from anode through aqueous environment (soil, see water etc.) toward steel structure.

12Schematic figure of SACP 13Schematic figure of ICCP 14

The current has to flow in a loop and therefore the surface of structure that can be protected is the submerged surface that received protection current from anodes.

15In case of inner pipe surface needs to be protected by CP, the anodes (usually ICCP anodes) have to be installed inside the pipe.Note that the anode spacing for internal protection of pipeline is limited. 16Consider this 17

Under protection

Surface area which receives insufficient protection current density, still corrodes. This insufficient cathodic protection is known as under protection.

Overprotection

Coating at surface area which receives excess protection current have a tendency to disbonding and blistering. In extreme case, i.e. when the environment contaminated by arsenic, antimony or sulphide, ions diffusion of atomic hydrogen into the steel may occur and results in hydrogen embrittlement. This excess cathodic protection current is known as overprotection.

Net protective current flows from electrolyte into the structure surface

19Pipe to soil potential -850mV vs. CSE

- 850 mV (CSE) with the CP applied

The full criterion states that adequate protection is achieved when a negative (cathodic) potential of at least 850mV with the CP applied.

Probably the most widely use criterion for determining an acceptable level of buried and submerged steel or cast iron structures. Strictly applicable only to steel in neutral environments, such as soil and seawater.

Voltage drops other than those across the structure to electrolyte boundary must be considered for valid interpretation of this voltage measurement.

The voltage drops resulted from current flow in the electrolyte (soil) are generally referred to as ohmic or IR voltage drops.

20(No Transcript) 21

IR voltage drop are more prevalent in the vicinity of an anode bed or in areas where stray current are present and generally increase with increasing soil resistivity.

For bare or very poorly coated structure, IR voltage drops can be reduced by placing the reference electrode as closed as possible to the structure.

For the majority of coated structures, most of the IR drop is cross the coating, and the measurement is less affected by reference electrode placement.

The IR voltage drop can also be minimized or eliminated by current interrupting all of direct current resources of the CP system and measuring the instantaneous potential.

22

Polarized potential of -850mV criterion

This criterion states that adequate protection is achieved with a negative polarized potential of at least 850mV relative to a saturated copper/copper sulfate reference electrode.

The polarized potential is defined as the potential across the structure/electrolyte interface that is the sum of the corrosion potential and the cathodic polarization.

The polarization potential is measured directly after interruption of all current sources and is often referred to as the off- or instant off-potential. The difference in potential between the native potential and the off or polarized potential is the amount of polarization that has occurred as a result of the application of the CP.

23In this location pipeline is under protection (not enough protected)Can be applied only for ICCP systems 24

Cathodic polarization 300mV below Ecor

This criterion states that adequate protection will be achieved when the interface potential (pipe to soil) is decreased 300mV (or 400mV when SRB is present) below its native potential. This criterion is referred to the first criterion and the potential of fresh steel structure is about -550mV vs. CSE.

25

Cathodic polarization 100mV during interrupted

This criterion states that adequate protection is achieved with a minimum of 100 mV of cathodic polarization between the structure surface an a stable reference electrode contacting the electrolyte.

To determine the magnitude of the shift as a result of the formation of polarization, one must first determine the native potential of the underground structure at test location before applying CP. The potential is then re-measured after the CP system is energized and the structure has had sufficient time to polarize. Typical of on potential is continuously monitored at one test location, and an off potential is made when there is no measurable shift in the potential reading for several minutes. The off potential is then compared with the native potential, if the difference exceeds 100mV, then the 100mV criterion has been satisfied at that location.

Cathodic polarization to a potential where Tafel behavior is achieved.

This criterion states that adequate protection is achieved with protection potential E1.

Protection potential E1. Note that E1 is gt than -850 mV. The method can be applied only for ICCP systemELog IE vs log I curve of a pipeline at certain location 28

Net protective current flows from electrolyte into the structure surface

This criterion qualitatively informs that the structure is being cathodically polarized which leads to reduce the corrosion rate of structure However this criterion cannot be used to evaluate the condition of the steel (pipeline).

It might be used to evaluate the present of stray current.

29(No Transcript) 30Each material has its protection criteria 31Influence of interface potential on the condition of steel of a coated pipelineIn many cases on cathodic protection of buried pipelines, the potential of the location around drainage point can reach 1.5V vs. CSE but it is still accepted. 32Base on technical and economical consideration cathodic protection is applied together with coating/wrappingCost of protectionCost for coating aloneNote that there is no coating that is free from porosity, and coating damage will normally occur during service.Cost of cathodic protection aloneMinimum protection cost0 bare metal100 33In this case cathodic protection only protect the exposed area, 34One of the coating properties related to cathodic protection current requirement is coating resistance/conductanceNote that the data is for a pipeline having certain diameter and length 35Sacrificial Anodes Cathodic Protection (Cathodic Protection with Galvanic Anodes)

The anode capacity is the current which can be produced by a stated weight of anode material, and its usually expressed in ampere hours/kg (Ah kg-1) Owing to the fact that the efficiency is less than 100, the capacity of anode in practice is normally less than the theoretical Faradaic capacity.

45

Example Anode capacity of aluminium

46HALF CELL POTENTIAL OF SEVERAL STANDARD ELECTRODES 47Formula to calculate total weight and number of anodes Total protection current requiredTotal weight of anodes requiredW is the total weight of anodes w is the weight of an anode chosen 48

Protection current available from single anode

E1 expected protection potential of structure

E2 is corrosion potential of a sacrificial anode is

Ran is resistance of single anode

Resistance of a slender anode (where the ratio of length to mean effective radius greater than 10)

In case for protecting submerged marine structures

? is seawater resistivity, L is the length of anode and reff is effective radius of anode.

49Effective radius of the anode is taken as a radius that is left after 40 of anode is consumed (utility factor is taken 80)Minimum number of anodes that has to be provided

For a horizontal anode, the anode resistance may be presented as 50Water resistivity can be obtained from this diagram provided that temperature and water density are known Given density 1020g/l and temperature 200C, then resistivity is 25 Ocm 51

Example

Given Ecor Al anode - 1.05 V vs. Ag/AgCl

Eprot of steel - 0.80V vs. Ag/AgCl

Anode dimensions 120x 6 x 6cm3

Water density 1020 g/l

Water temperature 200C

Then Water resistivity

Anode resistance

Anode output

52

If the ratio of length to mean effective radius is small (less than 110) then the resistance of anode can be determined as follow

where L is the length of anode (cm),

W is the width of anode (cm)

and T is the thickness of anode (cm)

Output is higher than that of slender anode, consequently the anode consumption is greater and the anode life is shorter.

53Formula for the resistance of a horizontal anode with an cross sectional area of up to 105 cm2, can be expressed in a graphical form. This graph is for determining anode resistance in seawater, different graphs are needed for different water resistivities. 54

For soil, the resistance of horizontal anode can be calculated using the following formula

where, Rh is the resistance of horizontal

anode to earth in ohms

? is the resistivity of backfill material

(or earth) in ohm.cm

L is the length of anode in feet

d is the effective diameter of anode in feet

S is the twice deep of anode in feet

Resistance for vertical anode can be calculated using formula above where ? is the resistivity of backfill material (or earth) in ohm.cm

55

Method for choosing the weight and number of aluminium and zinc anodes

For any specific situation, the total anode weight, the total current required and the number of anodes which will meet these current and weight requirements are calculated as follows

The wetted area of steel to be protected is calculated from drawings or direct measurement.

The total current IT (A) needed is

56

3. The following formula gives the weight of anode material required

were life is the design life in years (1y8760h) and UF is utility factor ( utility factor that usually taken is 80).

4. The minimum number of anodes required per structure is assessed from the following formula

Note the anode selected must satisfy both the total weight and total current output requirements as follows

Weight requirement No of anodes x individual net weight

Current requirement No. of anode x individual anode current out-put

57

Current densities for protecting steel

I.e. current density requirements for ships hull,

ballast and tanks

58

Total resistance of each anode to earth consists of the resistance of anode to the backfill material plus the resistance to the earth of backfill column itself.

The formulas can also be used to calculate the resistance of an impressed current anode. However one should consider in case of inert anodes are used, the effective diameter (or effective radius) in the formula has to be changed with the initial diameter (or radius) of anode because the diameter of anode is reduced very slightly.

59Physical shape of anodes for protecting marine pipes 60Physical shape of the anodes for protecting marine structures 61Zinc and magnesium anodes used in soil environment 62

Different shapes give different surface area to weight relationships, and this results in different shapes giving different current outputs for the same weight. This means that different shape have a different life for the same weight. In general the shape is chosen in order to give a certain current output for a certain weight and thus have a certain life.

In a number of other instances the anode shape is designed to conform with the shape and room limitations of the steel structure that it is design to protect.

63BACKFILL FOR SACRIFICIAL ANODES

In a soil environment, zinc or magnesium anodes that are used for a cathodic protection system have to be surrounded by a backfill.

Objectives

To decrease anode resistance.

To avoid anodes directly contact to soil. Anodes directly contact to soil may suffer localized corrosion or be passive. I.e. chloride decreases the efficiency of magnesium anode while anodes that directly contact to phosphate, carbonate or bicarbonate in soil can produced film which hinders further anode dissolution.

Decide the protection criteria and life time for the pipeline. This includes

- Criterion used for protection

I.e. pipe to soil potential has to be

- 850mV vs. CSE

- Cathodic protection system is designed for

T years

Technical data available related to cathodic protection of the pipeline have to be collected.

Pipe dimension - diameter

- length

- thickness

69

Specific resistance of Coating ? ohm.m2

Soil resistivity ? ohm.cm

Backfill resistivity usually 50 ohm.cm

Required protection current density ip mA/m2

Type of anodes used and its capacity (KAh/kg)

Driving voltage of anode ?E (volt)

3. Compute how much the total protection current is required

I0 p D L ip

4. Find the total weight of anode required

where, U is utilization factor usually 0.8 (80)

70

5. Find the dimension and weight of anode for this

protection system, w.

Note that heavier anodes are recommended to

be used in more conductive soil, while small

anodes are provided for high resistivity soil.

71

6. Calculate the number of anodes

7. Design the installation method and then determine the anode/groundbed spacing.

For groundbed

with single anode

For groundbed

with multiple anode

8. Compute the requirement of protection current

for the length of pipe of S or SG

Is p D S ip or IsG p D SG ip

72

9. Calculate the resistance of single

anode .

Resistance of anode (Rtotal)

Ranode to backfill Rbackfll to soil

I.e for a vertical anode

where Rv is the resistance of vertical anode,

l is length and r is radius.

The dimension of backfill is assumed to be constant.

73

Effective radius is calculated using the following formula

10. Determine the current output from a single anode/ a

groundbed

or

Rground bed is computed as follows

74

where F is the interference factor

and n is the number of anodes in one ground bed.

11. If for groundbed with single anode

Is Ia or

for groundbed with multiple anodes

IsG Igrounbed design is accepted.

If for grounbed with single anode

Is Ia or

for groundbed with multiple anodes

IsG Igrounbed

check the dimension of anode or distance between two anodes in a groundbed.

Groundbed with multiple anodes may need higher number of anodes.

75CASE STUDY 1

Protection criteria -0.85 V vs. CSE

Designed life time of CP 20 years

Data - pipe dimension diameter 14

thickness 0.5

length 10km

- Coating polyethylene tape, overlap 5cm

- Average resistivity of soil 2000 ohm.cm

- Required protection current density 0.5 mA/m2

- Anode used magnesium anodes

Anode capacity K 1200 Ah/kg

?E 0.7 volt

76

Length of anode 50 cm

Diameter of anode 12 cm

Length of backfill 80 cm

Diameter of backfill 20 cm

Design the SACP system for the pipeline

77Installation of single anode 78Usually it is recommended to install sacrificial anode below pipeline because backfill has always to contain water which is required for maintaining dissolution of sacrificial anode. 79

92(No Transcript) 93SACP needs high number of large anodes. Consequently large structure of rig has to be constructed.ICCP needs small number of light anodes. PROTECTION OF OFFSHORE MARINE OIL DRILLING RIG 94

Disadvantages

Can cause anodic and cathodic interferences

Can result in overprotection

Unsuitable for congested structures

Needs frequent inspection

Wrong polarization will damage the structure

Needs careful design

95Structures that can be protected by ICCP

PIPELINE (ONSHORE and OFFSHORE)

STORAGE TANK

STEEL PILES PIER

JACKET

REINFORCING STEEL IN CONCRETE

PLATFORM, RIG

SHIP HULL

Etc.

96Mixed oxide anode is also recommended 97Low tide levelinsulatorCONTACT BETWEEN STRUCTURE AND AN ICCP ANODE/ANODES HAS TO BE AVOIDED 98(No Transcript) 99Anode installation(It is recommended that the groundbed has to be at lower level than the pipeline)Recommended space distance between a pipeline and a groundbed is 150m 100Deep well installation It can be used for ICCP of cashing or when field for ground bed is limited 101

Materials used for ICCP anodes

Rare materials

Ferrous materials

Lead materials

Carbonaceous materials

Reactive non ferrous metals

Combination anodes

102Rare materials

Platinised tantalum

Platinised niobium

Platinised titanium

Platinised silver

Platinum metals

Dimensionally stabilized anode (DSA) titanium

103Ferrous materials

High silicon chromium iron

High silicon molybdenum iron

High silicon iron

Cast iron

Steel

Iron

Stainless steel

104Lead materials

Lead-antimony-silver

Lead/platinum bi-electrodes

Lead dioxide/titanium (lead dioxide on titanium substrate)

Lead dioxide/graphite

In using lead as an anode the formation and maintenance of a hard layer of lead dioxide (PbO2) is essential.

Pure lead fails to passivate and leads to chloride formation beneath the lead dioxide thus insulating the latter from the lead.

The carbonaceous backfill surrounding an anode is essential and serves a number of functions

1. To reduce anode resistance. (It has the effect of increasing the anode size which results in reduction of anode resistance to earth.)

2. To extend the service life of anode. (Most of the current is transmitted to the backfill from the anode by direct contact, so that the greater part of material consumption is on the outer edges of the backfill column, enabling the anode themselves to have an increase life.)

3. To provide paths for flowing gas produced from anodic reaction

116

Carbonaceous backfill is used for HSI, HSCI, graphite, mixed oxide anodes and others.

Backfill Material

Coal coke (650-800kg/m3) has met with the standard and can directly be used as backfill

Petroleum coke (700-1100kg/m3) has to be calcined before it is used as backfill material

Natural graphite granules or crushed graphite (1100-1300kg/m3) can be used

In practice, the coke consumption will be of the order of 0.25kg/A.Y

117

Typical coal coke specification

100 will pass through a 15mm aperture

85 will pass through a 10 mm aperture

15 will pass through a 5 mm aperture

Volatile matter 3.25 max.

Fixed carbon 78 min.

Ash 19 max.

Sulphur 1.2 max.

Resistivity (bulk) 55 Ocm max

The coke breeze should be thoroughly mixed with 5-10 by weight of slaked lime, to counter act the tendency to lose moisture by electro-osmosis. Calcium sulphate is sometime used in very dry condition.

118

Too much fines leads to over-tight compaction and gas blocking leading to gas polarization.

Flake graphite is not recommended as its tends to conglomerate and prevent gas emission.

Resistivity of carbonaceous backfills, Ocm

119Typical rectifier for cathodic protection 120Rectifier can be connected to a control circuit either to produce constant current output or constant potential output. 121Economics

Because both the cost of the rectifier and the cost of electric power consumed are contingent on the operating voltage of the system, it is desirable to keep the operating voltage as low as possible, for this reason a low resistance groundbed is desirable when it is economically feasible.

122Typical cost as function of number of anode in one groundbed

About PowerShow.com

PowerShow.com is a leading presentation/slideshow sharing website. Whether your application is business, how-to, education, medicine, school, church, sales, marketing, online training or just for fun, PowerShow.com is a great resource. And, best of all, most of its cool features are free and easy to use.

You can use PowerShow.com to find and download example online PowerPoint ppt presentations on just about any topic you can imagine so you can learn how to improve your own slides and
presentations for free. Or use it to find and download high-quality how-to PowerPoint ppt presentations with illustrated or animated slides that will teach you how to do something new, also for free. Or use it to upload your own PowerPoint slides so you can share them with your teachers, class, students, bosses, employees, customers, potential investors or the world. Or use it to create really cool photo slideshows - with 2D and 3D transitions, animation, and your choice of music - that you can share with your Facebook friends or Google+ circles. That's all free as well!

For a small fee you can get the industry's best online privacy or publicly promote your presentations and slide shows with top rankings. But aside from that it's free. We'll even convert your presentations and slide shows into the universal Flash format with all their original multimedia glory, including animation, 2D and 3D transition effects, embedded music or other audio, or even video embedded in slides. All for free. Most of the presentations and slideshows on PowerShow.com are free to view, many are even free to download. (You can choose whether to allow people to download your original PowerPoint presentations and photo slideshows for a fee or free or not at all.) Check out PowerShow.com today - for FREE. There is truly something for everyone!

presentations for free. Or use it to find and download high-quality how-to PowerPoint ppt presentations with illustrated or animated slides that will teach you how to do something new, also for free. Or use it to upload your own PowerPoint slides so you can share them with your teachers, class, students, bosses, employees, customers, potential investors or the world. Or use it to create really cool photo slideshows - with 2D and 3D transitions, animation, and your choice of music - that you can share with your Facebook friends or Google+ circles. That's all free as well!

For a small fee you can get the industry's best online privacy or publicly promote your presentations and slide shows with top rankings. But aside from that it's free. We'll even convert your presentations and slide shows into the universal Flash format with all their original multimedia glory, including animation, 2D and 3D transition effects, embedded music or other audio, or even video embedded in slides. All for free. Most of the presentations and slideshows on PowerShow.com are free to view, many are even free to download. (You can choose whether to allow people to download your original PowerPoint presentations and photo slideshows for a fee or free or not at all.) Check out PowerShow.com today - for FREE. There is truly something for everyone!